This invention is directed to a method of securing a synthetic plastic closure assembly to a synthetic plastic pail cover.
The current pressing need to eliminate plastic packaging materials from the solid waste stream has prompted an in-depth cost analysis of the most effective approach for getting industrial size plastic pails converted into post consumer regrind plastic resin. These pails, by and large, are in the five gallon size range and consist of a generally cylindrical vessel open at one end to which a cover is mechanically applied after filling. The cover, in turn, has a pouring opening to which a closure assembly is secured to enable subsequent decanting and reclosing. Pail closures normally employed in this environment are of the type shown in U.S. Pat. No. 4,568,006 dated Feb. 4, 1986 and consist of a plastic nozzle embodying a nestable pouring spout which threadedly receives an overlying screw cap. The nozzle is further provided with a circumferential sealing flange secured to a neck surrounding the pail cover opening by means of a rigid metal crimping ring. While this type pail closure construction has proven highly successful in the past, it has the serious shortcoming for present day needs of incorporating a metal component in an otherwise all plastic construction. The additional labor required to remove this metal ring so that the pail cover can be accepted into the plastic recycling process is a significant negative factor. This drawback has prompted the search for an effective alternative method of securing an all plastic closure assembly to the pail cover utilizing one of the known plastic to plastic bonding techniques. Although such known bonding techniques are numerous including spin welding, sonic welding, hot plate welding and induction welding. None have to date provided a completely satisfactory answer to the above described problem at hand.
To be successful such alternative method must produce a closure pail cover joint capable of withstanding all of the commonly accepted and closely regulated performance criteria for shipping hazardous liquid products. These criteria would include static and hydraulic internal pressure build up, drop test impact resistance at both high and low temperature extremes, environmental stress crack resistance under widely varying conditions and the like. These criteria, moreover, must be met with a high degree of consistency and repeatability. Each of the above enumerated known plastic to plastic bonding techniques has one or more serious limitations when applied to the instant situation. For example, most commonly employed plastic to plastic bonding techniques are largely ineffective when practiced with dissimilar plastic melt characteristics at the bond interface. That is to say, such methods in and of themselves will not provide the necessary joint strength and sealability. Induction welding, of course, requires the introduction of a metallic contaminant which is precisely what the invention seeks to avoid. Considerations of equipment cost and fabrication cycle times are also of importance in deriving a truly workable system.